Olfaction is important for human health, as it affects nutrition and quality of life, and is an early indicator of neurodegenerative diseases. Previously, we used the genetic power of Drosophila to show that the genetic architecture of olfactory behavior depends on dynamic epistatic networks of pleiotropic genes. Building on our previous discoveries, we now propose the following specific aims: (1) We will use a population genetics approach to characterize ligand specificities of odorant binding proteins (OBPs) in Drosophila melanogaster. OBPs are the first components of the insect olfactory system to encounter odorants and are encoded by a family of 50 genes. We will quantify responses to 12 odorants and resequence Obp genes in 300 wild-derived inbred lines, and perform association analyses. This will enable us to not only infer historical patterns of natural selection acting on individual members of this gene family, but also to obtain information about their ligand specificities and combinatorial ligand recognition by these proteins. (2) We will assess naturally occurring variation and environment-dependent plasticity in expression of chemoreceptors. We will use expression microarrays that represent all the chemoreceptors of D. melanogaster to assess natural variation in the expression of chemoreceptors and to assess modulation of the expression of the fly's chemosensory repertoire under different environmental conditions. (3) We will identify genes that contribute to natural variation in odor-guided behavior. We will use transcriptional profiling to identify genes that are differentially regulated between wild-derived inbred lines that are poor and strong responders to benzaldehyde. We will also generate recombinant inbred lines and hyper-recombinant mapping populations from duplicate high and low responding wild-derived inbred lines for high resolution QTL mapping. Our proposed studies will not only substantially extend our understanding of olfaction in Drosophila, but will also have implications for human health, as insect disease vectors rely on olfactory cues for host finding and OBPs could be potential targets for epidemiological pest control. In a broader context, genotype by environment interactions are a major confounding factor in association studies aimed at identifying polymorphisms that predispose to human disease. Olfactory behavior in Drosophila presents a well-characterized genetic model, in which one can begin to systematically unravel principles of genotype by environment interactions. This application proposes to study chemosensory behavior in a powerful genetic model system, the fruitfly Drosophila. Our studies are important for human health, as olfaction impacts human nutrition and quality of life, olfactory impairments are an early indicator of neurodegenerative diseases, and insect disease vectors rely on olfactory cues for host finding. In a broader context, interactions between genes and the environment are a major confounding factor in studies aimed at identifying alleles that predispose to human diseases and the well characterized genetic model of Drosophila enables us to begin to systematically unravel generally applicable principles of such genotype by environment interactions.